Electrical Properties of Membrane Phospholipids in Langmuir Monolayers

Experimental surface pressure ([Formula: see text]) and electric surface potential ([Formula: see text]) isotherms were measured for membrane lipids, including the following phosphatidylcholines (PCs)—1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC); 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC); and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). In addition, other phospholipids, such as phosphatidylethanolamines (represented by 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE)) and sphingolipids (represented by N-(hexadecanoyl)-sphing-4-enine-1-phosphocholine (SM)) were also studied. The experimental apparent dipole moments ([Formula: see text]) of the abovementioned lipids were determined using the Helmholtz equation. The particular contributions to the apparent dipole moments of the investigated molecules connected with their polar ([Formula: see text]) and apolar parts ([Formula: see text]) were theoretically calculated for geometrically optimized systems. Using a three-layer capacitor model, introducing the group’s apparent dipole moments (calculated herein) and adopting values from other papers to account for the reorientation of water molecules ([Formula: see text]), as well as the for the local dielectric permittivity in the vicinity of the polar ([Formula: see text]) and apolar ([Formula: see text]) groups, the apparent dipole moments of the investigated molecules were calculated ([Formula: see text]). Since the comparison of the two values (experimental and calculated) resulted in large discrepancies, we developed a new methodology that correlates the results from density functional theory (DFT) molecular modeling with experimentally determined values using multiple linear regression. From the fitted model, the following contributions to the apparent dipole moments were determined: [Formula: see text]; [Formula: see text] and [Formula: see text]). Local dielectric permittivity in the vicinity of apolar groups ([Formula: see text]) is much lower compared to that in the vicinity of polar moieties ([Formula: see text]), which is in line with the tendency observed by other authors studying simple molecules with small polar groups. A much higher value for the contributions from the reorientation of water molecules ([Formula: see text]) has been interpreted as resulting from bulky and strongly hydrated polar groups of phospholipids.